Buddhism teaches us to practice harmlessness. Would it be considered harmless to go around destroying non-'sentient' parts of nature? Can consciousness, particularly the pure awareness aspect, reside in such parts of nature (e.g. non-animals), or why not?
From the (non-dual!) Mahayana perspective, "mind" and "matter", "sentience" and "non-sentience" are definitely not two separate things, they are different aspects of the same.
For example, in his work called Bendowa Japanese Zen-master Dogen points out that:
In Buddhist philosophy, the body and mind are essentially one, and the essence and form are non-dual. So, don't separate the mind from the body. Why do you say that the body perishes while the mind is eternal? This is against the right law.
In addition to this, Dogen states in Shinjin-Gakudo:
All the worlds in the ten directions are the true human body itself.
and in Yuibutsu-yobutsu:
The whole world is one's own 'dharma-kaya'.
He also says:
Those [said to be] without the “mind” must also be sentient beings; this is because “all sentient beings” are the “mind.” Therefore, all that possess the “mind” are the sentient beings, and the sentient beings all have the Buddha-nature. Grass，trees, land ... are all “mind.” Since they are the “mind,” they are sentient beings；since they are all sentient beings, they have the Buddha-nature.
And then in Bussho:
When people hear the word “Buddha-nature” many scholars misunderstand it as akin to the atman of the heretics. It is because they do not meet the true person, nor do they see the real nature of their own selves, and furthermore it is because they do not encounter an authentic teacher. They unknowingly identify the function of human mind with the consciousness of the Buddha-nature.
While the Vietnamese Zen-master Thich Nhat Hanh says:
“If you are a poet, you will see clearly that there is a cloud floating in this sheet of paper. Without a cloud, there will be no rain; without rain, the trees cannot grow; and without trees, we cannot make paper. The cloud is essential for the paper to exist. If the cloud is not here, the sheet of paper cannot be here either. So we can say that the cloud and the paper inter-are. “Interbeing” is a word that is not in the dictionary yet, but if we combine the prefix “inter-” with the verb “to be,” we have a new verb, inter-be. The cloud and the sheet of paper "inter-are".
If we look into this sheet of paper even more deeply, we can see the sunshine in it. If the sunshine is not there, the forest cannot grow. In fact, nothing can grow. Even we cannot grow without sunshine. And so, we know that the sunshine is also in this sheet of paper. The paper and the sunshine inter-are. And if we continue to look, we can see the logger who cut the tree and brought it to the mill to be transformed into paper. And we see the wheat. We know the logger cannot exist without his daily bread, and therefore the wheat that became his bread is also in this sheet of paper. And the logger’s father and mother are in it too. When we look in this way, we see that without all of these things, this sheet of paper cannot exist.
Looking even more deeply, we can see we are in it too. This is not difficult to see, because when we look at a sheet of paper, the sheet of paper is part of our perception. Your mind is in here and mine is also. So we can say that everything is in here with this sheet of paper. You cannot point out one thing that is not here-time, space, the earth, the rain, the minerals in the soil, the sunshine, the cloud, the river, the heat. Everything co-exists with this sheet of paper. That is why I think the word inter-be should be in the dictionary. “To be” is to inter-be. You cannot just be by yourself alone. You have to inter-be with every other thing. This sheet of paper is, because everything else is.
Suppose we try to return one of the elements to its source. Suppose we return the sunshine to the sun. Do you think that this sheet of paper will be possible? No, without sunshine nothing can be. And if we return the logger to his mother, then we have no sheet of paper either. The fact is that this sheet of paper is made up only of “non-paper elements.” And if we return these non-paper elements to their sources, then there can be no paper at all. Without “non-paper elements,” like mind, logger, sunshine and so on, there will be no paper. As thin as this sheet of paper is, it contains everything in the universe in it.”
So now, given this information, what do you think, would it be considered harmless to go around destroying non-'sentient' parts of nature?
Maybe yes, or, "yes in some (or most) forms of Buddhism". Look at this answer for example:
A Bhiksu/Bhikkhu chopped down certain village sacred tree for building his little hut, the tree Deva's kid just playing around that tree accidentally being chopped off his finger. The tree Deva, devastated, went straight to complain to the Buddha... You see, here this Bhiksu is definitely unable to see, if, for example, he didn't get accomplishment in Dhyana/Jhana. Else he wouldn't chop off the Deva kid's finger, nor it's for the kid to make himself visible in normal circumstance; the Deva didn't file complaint to him directly since he wasn't able to see him.
I'm not sure whether that meets your definition: I think that there the deva is sentient (and has an immaterial body), the tree not considered "sentient" (iirc perhaps it's called three-factored), and the tree is somehow the deva's home.
I think this type of belief is known to the West as Animism ... and some people posit that Buddhist doctrine was admixed into indigenous (non-Buddhist) beliefs, such that forms (e.g. national forms, like Thai) include elements of both. I think devas and so on are a feature of all traditional forms of Buddhism -- I'm only unsure of whether they're associated with "nature".
As for "Would it be considered harmless to go around destroying non-'sentient' parts of nature?", you might like to read this (or other writings related to such a topic):
That's written by someone associated with "the Friends of the Earth" (which I think is an international organization of environmental activists, who are not necessarily Buddhist, so maybe biased somewhat and not a reliable editor of Buddhist doctrine).
As for what's "harmless" it would be an idea to check the Vinaya. I think that breaking (or killing) a living plant is not "allowable" to a monk, is a fault requiring confession -- "taking what's not given" might be an issue too.
I found that what I quoted above is a paraphrase of the story associated with Dhp 222 -- which may also be the origin story for the Vinaya rule against Bhikkhus cutting plants:
Once, a bhikkhu from Alavi wanted to build a monastery for himself and so he began to cut down a tree. The deva dwelling in that tree (rukkha devata) tried to stop him, saying that she and her infant son had nowhere to go. Failing to stop the bhikkhu she put her son on a branch, hoping that it would stop him from felling the tree. By then, the bhikkhu was already swinging his axe and he could not stop it in time and unintentionally cut off an arm of the child. Seeing her child being harmed in this way, the mother flew in a rage and was about to kill the bhikkhu. As she raised her hands to strike the bhikkhu, she suddenly checked herself and thought, "If I were to kill a bhikkhu, I would be killing one who observes the moral precepts (sila); in that case, I would surely suffer in niraya. Other guardian devas of the trees would be following my example and other bhikkhus would also be killed. But this bhikkhu has a master; I must go and see his master." So she went weeping to the Buddha and related all that had happened.
To her the Buddha said,
"O rukkha devata! You have done well to control yourself."
Then the Buddha spoke in verse as follows:
Verse 222: He who restrains his rising anger as a skilful charioteer checks a speeding chariot, — him I call a true charioteer; other charioteers only hold the reins.
At the end of the discourse the deva attained Sotapatti Fruition, and for her dwelling place she was offered a tree near the Perfumed Chamber of the Buddha. After this incident, the Buddha forbade bhikkhus to cut vegetation, such as grass, plants, shrubs and trees.